Last night experts were demanding why the Liberian-registered Braer was so close to the islands, which are classified as a protected zone. They were baffled by the explanation for the ship's engines failing.
Plans to contain the slick, which started to leak from the vessel last night, cannot be put into operation until the weather improves. Only westerly winds were stopping the oil spreading. However, light crude is more easily broken down than heavy crude from the Middle East, making it less damaging environmentally.
Insurance claims could run into undreds of millions of pounds, although some Lloyds underwriters did not expect it to rank alongside recent catastrophes, putting the likely cost closer to pounds 100m.
The Braer was carrying oil from Norway to Canada when she ran into trouble passing through the 22-mile gap between Fair Isle and Sumburgh Head. Winds were gusting to hurricane force.
At 5.20am Shetlands coastguards were alerted by the Maritime Rescue Co-ordination Centre in Aberdeen, which had received an emergency from the Braer. The crew said their engines had broken down 10 miles south of Sumburgh Head.
Ulthie Roldan, the chief engineer, said: 'There were problems with the generator and the boiler. The generator was not firing. The bad weather meant seawater had got into the fuel tank. We could not restart the engines.'
However, marine engineers said that it would be almost impossible for a vessel that size to be left helpless by fuel contaminated by seawater.
One engineer, who asked not to be named, said: 'I have been on board vessels when one tank has been flooded, but there are always at least two tanks and I have never heard of two or more being contaminated at the same time.'
Greenpeace in Norway last night called for an investigation into an incident at the Statoil terminal in Mongstad on Sunday when it is understood a pipe ruptured while the Braer was taking on fuel.
Statoil said that there was no connection between the incident and the ship's grounding, but Kalle Hesstvedt, an international co-ordinator for Greenpeace, said there were concerns that the accident could have led to water entering the vessel's fuel tanks.
After the Braer's SOS, Shetland coastguards alerted a helicopter at Sumburgh airport and two tugs at Sullom Voe. But they were forced to turn back and would have been too small to tow the tanker in such appalling weather.
The coastguard helicopter arrived over the drifting tanker shortly after 6.30am and half an hour later the winchman, Friede Manson, was lowered to take the first of the non-essential crew off. The wind was storm force 10, gusting to hurricane force 12.
With the ship drifting steadily towards the islands, Mr Manson brought up 16 sailors as the helicopter swayed precariously above the stricken vessel. An RAF helicopter took the rest.
At 10.15am an oil rig support vessel, Star Sirius, arrived after taking two-and-three-quarter hours to battle 30 miles. It was hoped that the 1,500-tonne vessel would be able to tow the tanker away from the rocks, then just three quarters of a mile away.
The Braer's Greek captain, Alexanderus Gelis, asked to go back on board with three crew and two marine pilots after hopes that currents might sweep the tanker out to sea were dashed.
They tried to fire rockets with lines attached from the Braer to the Star Sirius but missed.
The tanker then struck rocks at the foot of a 150ft high cliff at Garths Ness on the southern tip. Two minutes after it went aground three men still on board were winched to safety.
Mr Manson said: 'Time ran out. We winched off some crew and when she grounded there was just me and the captain and a Sullum Voe pilot still on board. The ship bumped and lurched all over the rocks. I was certainly very frightened.'
Lord Caithness, Minister for Shipping, said that the Government would meet the initial cost of the clean-up.
John Leach, an environmental health officer with Shetland Island Council, and colleagues watched helplessly yesterday: 'We had all the equipment, the restraining booms, the maps, but because of the prevailing weather conditions we could only watch.
'Our emergency plans are practised constantly and we have had offers of help and equipment from across the country. There has been a tremendous response, but until the weather changes we must wait.'
Roger Kohn, spokesman for the International Maritime Organisation, confirmed that the waters around the islands are classified as 'an area to be avoided'.
International collision regulations say a 10-mile protection zone is necessary to 'avoid the risk of oil pollution and severe damage to the environment and the economy of Shetland'. Regulations stipulate that all large ships should avoid the area.
Two years ago the IMO gave the zone legal sanction, which means vessels can be prosecuted for travelling within the area.
Many designers believe double-hulled vessels would be safer in 'lighter' collisions. They argue that a second hull may contain oil if the first is ruptured. But some designers think that gas could build up between the two skins, presenting a risk of explosion.
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